The invention is a method of using gravure printing or coating to produce a continuous gradient on a substrate, and in particular, a color gradient: on a transparent substrate. An example of a resulting product would be a tinted window film such as those used in buildings or automobiles that have a very dark color gradient at the top to act as a sun shield, and below which the color gradient becomes progressively lighter. Numerous other applications exist such as specialized lighting or optical fixtures, as well as applications for nontransparent coatings or nontransparent substrates.
In gravure printing, a roll (i.e., a cylinder) carries a multitude of small openings on its surface called cells, and rotates in contact with both an ink supply and a web (such as paper or polyester) to be printed. The cells take up the ink from the supply and transfer it to the web. Gravure is quite economical for large scale printing or coating applications and thus is a favorable technique if an appropriate printed product results.
In most gravure processes, the cells are flooded with ink, after which a doctor blade wipes the surface of the roll (or in some cases a plate) of excess ink. Thus, the depth and size of each cell determine the amount of ink (or other fluid) that is available to be transferred to the printed (or otherwise coated) surface.
A cell has two size components. The first is its cross-sectional area, which is often described by the number of lines of cells per inch (line density) along the gravure roll. The higher the line density, the smaller the individual cells. The second component is the cell's volume, which depends upon both the cross-sectional area and the cell's depth on the roll.
The conventional gravure method for producing a color gradient is to reduce the volume of the individual cells from start to finish around the circumference of a gravure cylinder. Bigger cells pick up and distribute more ink, and smaller cells correspondingly pick up and transfer less ink. Thus, a web printed by such a roll will exhibit a pattern of more ink decreasing to less ink; i.e., a color gradient.
The conventional technique, however, tends to produce several distinct portions of the color gradient. In general terms, the first portion is produced by the largest volume cells and is essentially solid in appearance. As the cell volume decreases, the next portion appears as a continuous field of color with discontinuous unprinted dots. As the pattern progresses, the appearance changes to an unprinted continuous field with discontinous printed dots. A final portion appears as a continuous unprinted field.
An optical problem arises, however, when such gradients are produced by conventional gravure processes on transparent substrates. Specifically, the intermediate portions of the gradient (the printed field with unprinted dots) produce an optical haze on a transparent substrate. For example, if a transparent colored portion is printed on a colorless polyester substrate, the darkest colored portions, the distinct dot portions, and the uncolored portions will be optically clear. The unprinted dot portions, however, tend to be hazy. Such hazy portions are undesired or unacceptable for many purposes.